The well-known Rankine vapor power cycle or its modifications (for example, reheat and regenerative cycles, dual pressure cycle, and cogeneration cycles) are currently used to produce electrical power. In these systems, steam is generally the working fluid of choice because of its easy availability, chemical stability, and relatively low cost. During the cycle, heat is added to the system to generate steam at high pressure which in turn is expanded through a turbine to generate power.
Gas turbine power cycles are analogous to vapor power cycles in that the individual processes are steady flow processes carried out in separate components. The working fluid in a gas turbine power cycle, however, is generally air or the products of combustion of fuel and air. Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. If a fuel is used in such a system, heat is generated within the system by the fuel being combusted in a compressed air stream, and the resultant combustion products gas stream is expanded through a gas turbine to produce power.
The metallurgical temperature limit on the gas turbine blades necessitates a gas turbine operation with a very high air-to-fuel ratio. In a conventional gas turbine system, the nitrogen in the feed air and the excess oxygen present in the combustion products gas stream act as heat sinks and thereby lower the temperature of the combustion products gas stream. As a result, the exhaust gas stream from the gas turbine power cycle contains excess oxygen in which additional fuel could be burnt. These hot exhaust gases could also be used to preheat the compressed feed air or may be used to generate steam that can be employed in a vapor power cycle. The latter combined power plant is generally referred to as a COGAS plant.
It is also possible to recover some or most of the oxygen not used to support combustion from a gas turbine cycle using ion transport membrane technology. Most oxygen generating systems utilize cryogenic gas separation methods (high purity, large scale) or membrane and adsorptive separation techniques (90-95% purity, small-medium scale). Conventional non-cryogenic bulk oxygen separation systems, for example, organic polymer membrane systems, are typically very power intensive, and are usually suitable only for the production of small quantities of oxygen-enriched air (for example, 50% oxygen). Although some of these conventional processes recover a part of the power utilized in producing the product, they do not produce any net power. In addition, conventional oxygen separation processes operate at low temperatures (less than 100.degree. C.), and do not benefit significantly from integration with a power generation process.
An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure. Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. One of the larger problems is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 700.degree. F. (370.degree. C.), generally in the 800.degree. F. to 1850.degree. F. (425-1000.degree. C.) range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, and U.S. Pat. No. 5,733,435, entitled Pressure Driven Solid Electrolyte Membrane Gas Separation Method, which are both hereby incorporated by reference to more fully describe the state of the art. The elevated temperatures of operation, however, make ion transport processes well suited for integration with high temperature processes such as vapor-based, gas-based, or combined power cycles.
Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product Oxygen from Turbine Power Generation, relates to a process for generating net power using a combustion turbine, accompanied by the recovery of by-product oxygen-enriched gas. Air is compressed and heated, at least a portion of the air is combusted and a portion of the oxygen is removed from the air or combustion effluent using an air separator. The oxygen lean combustion effluent is expanded through a turbine to produce power. In an alternative embodiment, the effluent from the turbine is used to produce steam to generate additional power. In this process, the type of fuel is generally limited to "clean" fuels such as natural gas, oils, or synthesis gas.
Chen, U.S. Pat. No. 5,035,727, entitled Oxygen Extraction from Externally Fired Gas Turbines, relates to a process for recovering high purity oxygen from an externally fired power generating gas turbine cycle. While this process is similar to Hegarty (described above), Chen differs in the use of an externally fired gas turbine so that other types of fuels such as coal or biomass may be used.
Chen et al., U.S. Pat. No. 5,174,866, entitled Oxygen Recovery from Turbine Exhaust Using Solid Electrolyte Membrane, and Chen et al., U.S. Pat. No. 5,118,395, entitled Oxygen Recovery from Turbine Exhaust Using Solid Electrolyte Membrane, both relate to processes for extracting high purity oxygen from gas turbine exhaust streams by passing the gas turbine exhaust over an oxygen ion conducting membrane. In these processes, the oxygen separator employing an oxygen ion conducting membrane is placed downstream of some or all stages of the gas turbine, instead of upstream as in earlier patents. An electrically-driven ion transport unit is proposed when the turbine exhaust pressure is low. The exhaust stream from the oxygen separator is optionally expanded through an additional gas turbine stage.
Kang et al., U.S. Pat. No. 5,562,754, entitled Integrated High Temperature Method for Oxygen Production describes oxygen production by an ion transport membrane where the ion transport separator is located between two independently controlled direct, i.e. involving combustion, or indirect heating units. The permeate side of the ion transport membrane may be swept with steam. A stream of oxygen-containing gas preferably is heated in a direct-fired combustor, passed through the retentate zone of the ion transport membrane, and then directed to a gas turbine to generate power. This non-permeate stream is then discarded as exhaust.
Kang et al., U.S. Pat. No. 5,565,017, entitled High Temperature Oxygen Production with Steam and Power Generation, relates to a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated and steam is added. Water is added to the retentate gas stream from the ion transport module prior to the gas turbine to increase the mass flow in the turbine. This permits the ion transport module and the gas turbine to each operate at its optimum temperature.
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, describes compression and heating of feed air in a first heating step (using heat exchanger and combustor) before passing the heated, compressed air through an oxygen separator employing a mixed conducting oxide. The retentate gas stream from the ion transport module is heated in a second heating step before expanding it through a gas turbine to recover power. The hot exhaust gases from the gas turbine are used to produce steam that is expanded through a gas turbine to generate additional power. In these processes, the operating temperatures of the ion transport module and the gas turbine are independently maintained by controlling the rate of heat addition in the first and second heating steps.
Russek et al., U.S. Pat. No. 5,753,007, entitled Oxygen Production by Ion Transport Membranes with Non-Permeate Work Recovery, describes an integrated system for the recovery of oxygen and useful work. Hot non-permeate gas is recovered from the cathode side of an ion selective oxygen transport membrane. The non-permeate gas is cooled to near ambient temperature in a heat exchanger and expanded to a lower pressure generating shaft power which can be put to use, for example, to drive pneumatic tools.
None of the referenced patents have addressed co-production of nitrogen, the integration of ion transport membranes into Rankine power cycles and/or contemplate purging the permeate side of the ion transport membrane with combustion products and or steam, or elevated pressure steam and recovering oxygen and nitrogen at elevated pressure as taught by co-filed application entitled Solid Electrolyte Ionic Conductor Oxygen Production with Steam Purge, U.S. Ser. No. 08/972,410 by Prasad et al. now U.S. Pat. No. 5,935,298. The prior art has taught that ion transport membranes can be used to recover part of the oxygen not required for combustion from the compressed air stream in gas turbine cycles, however, this is accomplished at the expense of compressing additional feed air to replace the oxygen removed together with the capital costs associated with the oxygen removal system.